Self-Shielding Copper Substrate Neutron Supermirror Guides
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Self-Shielding Copper Substrate Neutron Supermirror Guides P. M. Bentley1,5, R. Hall-Wilton1,3,4, C. P. Cooper-Jensen1, N. Cherkashyna1, K. Kanaki1, C. Schanzer2, M. Schneider2, P. B¨oni2 E-mail: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] 1 European Spallation Source ERIC, Box 176, 221 00 Lund, Sweden 2 Swiss Neutronics AG Neutron Optical Components & Instruments, Br¨uhlstrasse 28, CH-5313 Klingnau, Switzerland 3 Universit`adegli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy 4 University of Glasgow, Glasgow, United Kingdom 5 Anderson Butovich Ltd. United Kingdom Abstract. The invention of self-shielding copper substrate neutron guides is described, along with the rationale behind the development, and the realisation of commercial supply. The relative advantages with respect to existing technologies are quantified. These include ease of manufacture, long lifetime, increased thermal conductivity, and enhanced fast neutron attenuation in the keV-MeV energy range. Whilst the activation of copper is initially higher than for other material options, for the full energy spectrum, many of the isotopes are short-lived, so that for realistic maintenance access times the radiation dose to workers is expected to be lower than steel and in the lowest zoning category for radiation safety outside the spallation target monolith. There is no impact on neutron reflectivity performance relative to established alternatives, and the manufacturing cost is similar to other polished metal arXiv:2104.02453v2 [physics.ins-det] 16 Apr 2021 substrates. Keywords: Neutron optics, metal substrate, copper substrate, supermirror, neutron shielding 1. Introduction With the imminent delivery of the first copper substrate neutron guides to the European Spallation Source (ESS), currently under construction in Sweden, it is timely to briefly Self-Shielding Copper Substrate Neutron Supermirror Guides 2 report on the rationale for, and development of, this new technology, and compare it to established alternatives. Neutron guides are solid tubes, usually of rectangular cross section. The optimisation of the geometry of these devices is a very broad topic and out of the scope of this paper. Here it is sufficient to note that instrument angular resolution, transport efficiency and background rejection often — but not always — tend to favour a channel width in the region of 4-6 cm, and due to mechanical stability requirements the substrates themselves are typically around 1 cm thick. On the internal surfaces, neutron mirrors are deposited to maximise the transmission of neutrons to the experimental stations. This allows the placement of the instrument remotely from the neutron source, some 10s or even 100s of metres away, reducing instrumental background and facilitating safe physical access designs. Historically, single, smooth metallic layers were used as neutron mirrors, but modern neutron guides use alternating Ni-Ti layers known as “supermirrors” [1]. In a typical neutron source, one has a means of neutron production (fission, fusion, or spallation) and a volume of temperature-controlled, strongly-scattering material known as a moderator. The optical interface between the neutron guides and the region within a few metres of the moderator is often known as the “beam extraction” area, and specific technology exists to improve the instrument performance. The neutron optics work in the beam extraction area carries greater technical challenges due to: • The energy produced along with neutrons creating a large heat load • A high radiation environment • A need for good reflective properties at large angles near the source High albedo, rad-hard materials are used around the moderator (e.g. beryllium, with some recent interest in nanodiamond, particularly for cold wavelengths [2]) where the grazing angles are large. At distances typically 1-2 metres from the moderator, the grazing angles are below the critical angles on supermirrors, but that still leaves heat- and radiation-loads that must be carefully managed. For the neutron guides entering the beam extraction area, polished metal [3] and glass-ceramic [4] optics have existed for some time and offer a robust, long- lifetime solution for high radiation environments. Indeed, irradiation tests indicate that supermirrors on metallic substrates do not show any degradation to a cumulative neutron fluence of 9 × 1019 n cm−2 [5]. This is comparable to sodium float glass (∼ 1 × 1020 n cm−2), and orders of magnitude higher than boron-containing glass substrates (∼ 1 × 1018 n cm−2 for some borkron variants and ∼ 1 × 1016 n cm−2 for borofloat). These fluences are integrated over the entire source spectrum, based on studies at the reactor source of the Institut Laue-Langevin (ILL, Grenoble, France) and the spallation source of the Paul Scherrer Institut (PSI, Villigen, Switzerland). They are assembled from shapes very similar to the glass guide variants, but instead of bonding by adhesives they are bolted together. In addition to improved lifetime, they also have the potential to enhance the fast neutron and gamma ray shielding Self-Shielding Copper Substrate Neutron Supermirror Guides 3 properties of the guide system, by increasing the density of the material immediately outside the supermirror channel. With the preceding technology, there is always a gap of several centimetres between the supermirror surface and the bulk shielding material, to allow for glass substrates, adjustment, and vacuum housings. The new idea here is to bring dense shielding material into direct contact with the supermirror. This can correspondingly reduce the total volume of shielding needed downstream through geometrical considerations, by placing the most effective shielding in the key locations where it will have the greatest impact. It is important to note that the shielding effect is in the longitudinal direction by virtue of the long line integral for low divergence beams, and thus the advantages are in the far-field sense. The transverse shielding effect is minimal, since the guide substrates themselves are only ∼1 cm thick and perhaps 2 metres of heavy shielding is required in the beam extraction area; indeed locally there should be an enhanced gamma production from neutron capture. Further benefits of metal substrates over glass are improved thermal conductivity, structural properties and robustness, which could allow the guides to be placed close to the neutron source without thermal damage occurring. Of particular interest is using these for guide inserts for in-monolith beam extraction from the target-moderator region at spallation sources. However, regions further out where fast neutrons are present may benefit from strategically placed effective shielding as well. Inspiration for the use of copper as a shielding material for fast neutrons at spallation sources came in part from experience of its use at other accelerator facilities and in high energy physics experiments. The possibility for its application here is motivated by the same advantages: its shielding effect for fast neutrons, thermal conductivity and structural properties. A couple of examples are given below: In the Large Hadron Collider [6] the area with the most intense radiation environment is around the experimental interaction points where the two beams collide. To protect the delicate superconducting accelerator equipment from such intense radiation and thermal loads, there are two key protective elements: the TAS and the TAN. The Tertiary Absorber of Secondaries (TAS) is a 1.8m long block of copper weighing around 2 tonnes and located at ∼20m from the interaction point, which blocks high energy particles from exiting the experimental cavern into the LHC tunnel; and the Tertiary Absorber of Neutrals (TAN) [7], which is a 3.5m long copper block, designed to absorb neutral particles (neutrons and pions) at about 140m from the interaction point, which can have energies up to the beam energy (7 TeV). The hadronic calorimeter of the CMS instrument is made from brass [8], whose stopping power for high energy particles compares favourably with the steel calorimeter on ATLAS [9]. Unpublished concepts from JPARC were also influential, where Cu had been used in the collimation of instruments for the same reasons [10]. This manuscript looks at using copper for the multifunctional purpose as substrate to the neutron supermirror guide, thermal transport and radiation shielding. Cost prevents deploying copper shielding liberally, but in some targeted areas it would be Self-Shielding Copper Substrate Neutron Supermirror Guides 4 Figure 1. Comparison of different spallation target shielding concepts at various leading facilities around the world, both operational and under construction. The ESS has two concepts, a lower part and an upper part. A design change reduced the steel shielding for the lower parts physically below the neutron beam port level to save money. ideal. 2. Shielding at Spallation Sources The initial motivation for the copper guides, and copper shielding in general, was to reduce fast neutron background signals on the instruments of the ESS, based on investigations of challenges faced by similar, operational facilities [11]. Shielding is a significant fraction of a neutron facilities cost, both in terms of shielding the source and shielding the instruments. Indeed, shielding and